I'm still not sure why your HUD is blank (do you mean totally blank, or it's just the standard HUD with no weapon stuff?), but try this:

I assume you know how to operate weapon, but just in case you aren't totally sure, here's the steps: press Shift+1 to open the control panel, and flip the master switch (far left) to turn on weapon. Everything from there is easy, just navigate to the weapon category (i.e. missile, gun, rocket) and press Ctrl+Shift+7 to cycle through the weapons in each category (for the Hydra rockets--the one with 120 ammo--you should press Ctrl+Shift+8 twice to stop it from firing a ton of rockets at once and freezing the sim).

Okay, now that we're through that, Here's my advice: if your missiles don't fire, it's probably because your firing keys/joystick buttons conflict with simulator commands. For instance, on my computer, Spacebar activated mouse look-around, so naturally I could not use that for firing. Joystick Button 1 on my joystick was break, so that could not be used either. Read the SDK (download the pdf from captain sim) and find out how to re-map keys in the CSweapon.xml file (it's located in the CS_F18 folder in SimObjects and the folders of whatever other planes you installed weapon on--you'll have to re-map for each)--I changed it to the O key and Joystick Button 3, neither of which had previously mapped commands.

Right. That's a lot of stuff, most of which you probably won't need, but I just want to include as many possible solutions as possible  ;). If you could provide a screenshot of the virtual cockpit with the weapons panel open, that would be great, because right now all we're going off of is description (which you've provided plenty of  :)), and an actual picture might clearly show the problem.

Cheers,
boeing247